Technical Field
The present disclosure relates generally to nuclear magnetic resonance (NMR) logging and, more specifically, to techniques for correction of motion effects in NMR logging.
Background Information
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the subject matter described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, not as admissions of prior art.
Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, nuclear tools, acoustic tools, and nuclear magnetic resonance (NMR) tools, though various other types of tools for evaluating formation properties are also available.
Early logging tools were run into a wellbore on a wireline cable after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, as the demand for information while drilling a borehole continued to increase, measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools have since been developed. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, NMR distributions, and so forth. MWD and LWD tools often have characteristics common to wireline tools (e.g., transmitting and receiving antennas, sensors, etc.), but MWD and LWD tools are designed and constructed to endure and operate in the harsh environment of drilling.
In LWD operations, the drilling process can induce a complex lateral motion whose amplitude and frequency spectrum can depend on a number of parameters. For instance, the motion can have random and periodic components depending on various parameters, such as weight-on-bit (WOB), RPM, stabilizer size, torque-on-bit (TOB), and/or inclination, to name just a few example. Further, the motion may also differ based on the drilling path orientation/direction, i.e., vertical drilling and horizontal drilling may yield different induced motion behavior.
NMR tools using in well logging typically measure, among other things, relaxation times, such as transverse relaxation times (T2), of formation fluids, which can range from a fraction of a millisecond to several seconds. With respect to NMR logging tools, typically, an excitation slice is determined by an excitation bandwidth and a received slice is determined by a receiver bandwidth. A sensitive region may be determined based upon the smaller of excitation bandwidth (usually depends on available RF power) and receiver bandwidth. Essentially, the sensitive region is the overlap between the excited slice and the received slice, usually having the shape of a concentric shell. If an NMR logging tool moves by a sizeable fraction of the excited slice (typically having a thickness on the order of 1 centimeter) during tool operation, the resulting measurements can have reduced accuracy. As an example, the influence of tool motion can appear as an additional signal decay that makes an apparent T2 appear shorter than its intrinsic value (e.g., expected value if no induced motion were present). This can result in an under-estimation of permeability, which is used to evaluate formation productivity. Accordingly, addressing the effects of tool measurements that can be caused by the above-described types of induced lateral motion during LWD drilling applications is a challenge for the industry. It would be desirable to have a technique for removing or otherwise compensating for such motion effects.